Anodizing method and solutions



3,112,259 7 V ANQDEZHNG METHOD AND SOLUTKQNS Henry Walker, 49% Bluebell Ave, North Hollywood, Los Angeles, Calif. No Drawing. Filed Apr. 26, 1961, Ser. No. 1%,544 6 Qlaims. (8i. Elle-28) This invention relates to the surface treatment of aluminum and aluminum alloys and, more particularly, to an improved method and means for forming thereon a coating layer, namely, comprising oxides.

This application is a continuation-in-part of United States patent application by this inventor, Serial No. 746,666, filed July 7, 1958, and now abandoned, for Anodizing Solutions.

The property of insulation of oxide coatings on aluminum and aluminum allow articles has caused consideration to be given to the use of anodized aluminum for magnet wire. However, because of the poor flexibility, poor moisture resistance, poor craze resistance, and long processing time, the use of the anodized aluminum for such purposes has been restricted. The anodic film created by the usual commercial anodization process is limited to an elongation of 0.3 to 0.4%. Beyond this percentage, elongation crazing occurs. Another problem which arises is that crazing occurs at elevated temperature conditions. For example, films formed by hard anodizing will begin to craze as soon as the piece is Withdrawn from the cold processing bath. As the material warms up to room temperature, the crazing may not be visible; however, if the part is held near ones ear, the cracking of the coating can be heard. Another defect of the prior art anodizing process is the porosity of anodized films. This makes the insulation properties of the film dependent on relative humidity. Under conditions of high humidity, the insulation resistance of crazed wire and strip will gradually decrease and eventually a dead short may develop.

An object of this invention is to provide a method and means of producing a flexible anodizing coating.

Another object of this invention is to provide a flexible and noncrazing, or a sealed, oxide coating for aluminum.

Yet another object of the present invention is the provision of novel and useful anodizing solutions whereby there may be obtained an oxide coating on aluminum having the desired improved characteristics.

These and other objects of the invention are achieved by immersing aluminum in a cleaning bath and thereafter in one or more anodizing solutions, each of which will provide different desired properties for the finished anodizing coating. The last anodizing solution, or finishing solution, hardens the oxide layer, and improves the thickness, flexibility, and dielectric resistance thereof. In addition to these solutions employed in the embodiment of the invention, an electrical current density is applied in all baths up to 300 amperes per decimeter squared. As a result of both the high-current density and the novel solutions, the processing time for applying an oxide coating has been reduced from an average of 30 minutes to 15 seconds.

With the foregoing and other objects in view, which will appear as this description proceeds, the invention herein resides in the methods, processes, and compositions used therein and hereinafter described and claimed, it being understood that changes in the precise embodiment of the invention herein disclosed may be made without departing from the spirit and scope of the invention.

As is well known, an oxide film forms immediately on aluminum when the metal is exposed to air. This film protects the base metal from further oxidation, and thus tends to restrict its own growth. Since this film is extremely thin, its dielectric properties are negligible. The formation of this oxide film can be accelerated and the thickness increased by forced oxidation. Forced oxidation can be brought about chemically or electrochemically. The oxide film that is obtained chemically has not become of value to the electrical industry because its formation in useful thicknesses is relatively slow and because the film is soft and powdery. The oxide film that is formed electrochemically is known as an anodic film, because the aluminum is used as the anode in the process. Anodizing of aluminum has been studied by many investigators and is well understood. It is known that a thin, compact, highly dielectric film, known as a barrier layer, is formed at first and that this barrier layer resists the passage of additional current. When the anodizing current overcomes the resistance of the barrier layer, the film changes its characteristics. This change is manifested by the formation of pores in the oxide layer. The dissolution action of the electrolyte causes the pores to grow in size, which then permits the electrolyte to penetrate the pores and cause further oxidation. This further anodization then results in the formation of a new barrier layer.

The characteristics of the anodic film can be control-led by varying anodizing conditions, electrolyte, alloy, and, to a certain extent, the shape of the article being treated. Important factors in anodizing are time, temerature, and current density. These can be used to control film density, thickness, and porosity. Many types of electrolytes with and without additives have been and are being used to create ano-dic films. Use of proper electrolyte systems, coupled with conditions of operation, allows control over the film density, hardness, porosity, and appearance. The choice of alloy is not an important factor in anodizing electrical conductors. However, variations in alloys can sheet the dielectric properties and the appearance or" the anodic coating. In accordance with this invention, at least three baths are employed for processing wire or dielectric strips. The first is a cleaning bath; the second is an anodizing bath; and the third is a finishing bath, in which further anodizing occurs. Electrical current is caused to flow in a manner so that the aluminum wire, or strip, is the anode in the cleaning,

anodizing and finishing baths. Thus, in each bath an oxide layer is provided, the characteristics of which contribute to the finished product. Thus, in the cleaning bath an anodic layer film is deposited which has large pores. in the second anodic bath a film is deposited between the metal and the first layer film (which is pushed up), which is in general harder and less porous. The third anodic bath provides another layer of film between the metal and the second layer, pushing the first two up. Thus, one film having different layers is provided and each bath, including the cleaning and finishing baths, are employed in this process. Any well-known techniques may be employed for applying the electrical current; however, the arrangements shown, described, and claimed in my application for Anodizing Apparatus and Method, filed May 10, 1957, bearing Serial No. 658,292, and now abandoned, is preferred. In view of the nature of the solutions employed, it is possible to utilize current densities up to 300 amperes per decimeter squared in processing the aluminum. The limitation on the amount of current which can be applied is determined by the ability to maintain the temperature of the solutions employed herein within the desired temperature range. This is easily done, using amperes per decimete-r squared. However, any desired current density may be employed, since the current density plus the time the current is applied determines the thickness of the desired film. In accordance with this process, it is 0 possible to obtain a one-mil-thick film in 15 seconds. Normally, a one-mil-thickness film needs 45 minutes.

Any known precleaning solution may be used. The wire or strip after such precleaning can then be immersed in the first cleaning anodic cell. The cleaning solutions may be the usual cleaning solutions employed, but it is preferred that some oxalic acid, sulfuric acid and a wetting agent added for proper dispersion be present in the cleaning solution. It should be noted that although cleaning takes place in this solution, the current applied is such as to cause anodization to occur, also. This contributes to the speed of the process, as well as to the quality of the finished product. The anodizing solution will include sulfuric acid from 10 to 40% by weight, oxalic acid from 2 to by weight, glycerine from 2 to 20% by weight, methyl cellulose from 2 to by weight, oleic acid 0.5 to 5% by weight, and up to 1% by weight of a wetting agent. The remainder of this solution may be water. The proper-ties given by the various chemicals in solution are as follows: The glycerine acts as an inhibitor to produce higher plasticity and smooths the surface. The oxalic acid decreases the solubility of the metal in coating and increases the density of the film and also acts to preserve the life of the solution being employed. The methyl cellulose acts to harden the pores of the oxide coating, thereby eliminating the powdery qualities which are so characteristic in any anodizing, when high-temperature electrolytes are used, or a high current density. The oleic acid is a soap-forming substance which helps to improve the abrasion and wear-resistance qualities of the film and acts as a permanent lubricant thereof.

The thicker the anodizing film which is deposited by the anodization, the more spongy and the softer the outer layer becomes. The finishing tank is employed to harden the outer layer to improve the flexibility thereof, as well as its dielectric resistance. The finishing solution will contain 10 to 40% of sulfuric acid by weight, 5 to 10% disodium phosphate or sodium phosphate. This serves to improve the flexibility of the film. One-half to 3% of methyl cellulose may be employed, which serves to harden the upper pores. In place of the methyl cellulose, beryllium sulphate may be employed. However, the methyl cellulose is preferred, since it is much more inexpensive. As previously pointed out, electrical current is applied in a manner to maintain the aluminum as the anode in al the solutions. Either alternating current or direct current, or preferably a combination of both, may be employed. However, the current density which is applied is made high to increase the speed of deposition of the oxide coating. In View of the low resistivity of the solutions provided in accordance with this invention, a high-current density may be made to flow in the solutions.

It may be desirable to obtain different qualities for the oxide layers employed on aluminum, in which event any of the known anodizing soutions may be employed. However, by using as the last step of the method of treating the finishing solution or bath, whose composition has been given, superior flexibility, hardness, and dielectric resistance properties are given to the oxide coating.

For the purpose of producing foil strip continuously, a plurality of anodizing baths may be employed. After the cleaning solution, the strip may be immersed in a solution containing from 10 to 50% of sulfuric acid and one of magnesium chloride or sodium chloride, up to 2% by weight, and up to 10% by weight of tglycerine. Thereafter, in order to harden the film obtained, the strip may be immersed in an anodizing solution comprising 10 to 50% by weight of sulfuric acid and 0.5 to 3% by weight of methyl cellulose or beryllium sulphate. Thereafter, the aluminum foil or strip may be immersed in a finishing solution. It should be noted that the electrical current is applied to these solutions in a manner so that the foil or strip is the anode in the cleaning, anodizing and finishing solution. However, alternating current is preferred in this process. The aluminum strip will be the anode with alternating current durin the positive half cycles of this current. Another arrangement of anodizing solutions followed by the finishing both may be to im merse the aluminum foil or strip after the cleaning bath into a 'bath containing 10 to 50% by weight of sulfuric acid and a maximum of 5% by weight of oxalic acid, plus a wetting agent. Thereafter, the strip may be im= mersed in a bath comprising 10 to 50% by weight of sulfuric acid, up to 2% of a chloride selected from mag nesium chloride or sodium chloride, and 10% of gly-' cerine. Next, the strip or foil is immersed in a finishing bath, as described above. The result is that the strip or foil has a thicker film than before, which has a hard surface.

In order to obtain softness and flexibility for the anodizing film, the foil or strip may be fed from the clean ing bath into a solution of sulfuric acid 10 to 50% by weight and oxalic acid up to 5% by weight, plus a wetting agent. Thereafter, the foil or strip may be fed to the finishing bath. It may also be desirable to employ all five of the baths mentioned, namely, the cleaning bath, followed by the first anodizing bath containing the sulfuric acid and a chloride selected from magnesium chloride or sodium chloride, followed by the anodizing bath contain ing the sulfuric acid and oxalic acid, followed by the anodizing bath containing the sulfuric acid and methyl cellulose or beryllium sulphate, followed by the finishing bath.

The temperatures at which the anodizing process is carried out in accordance with this invention can affect the film. The lower the temperature, the harder the film will be; the higher the temperature, the softer the film will be. The preferred temperatures for the anodizing process are between 30 and F., the finishing bath should not have a temperature in excess of 70 F, and the cleaning bath should have a. temperature which should not exceed F. The preferred temperature for the anodizing baths is on the order of 40 F. Using the process described herein, 10 mil wire was made having an insulation resistance of 300 10 megs, and after 40 hours at a temperature of 600 C., the insulation resistance remained at 87 megs. The oxide coating was so very flexible that the wire could be wound on spools or solenoids several times the wire diameter without fracturing or crazing the coating. After 40 hours of treatment at 600 C., the dielectric breakdown value for the aluminum wire anodized in accordance with the method described herein was on the order of 325 volts. The oxide coatings having the desirable qualities are produced in accordance with this invention by successively immers ing the aluminum metal in one or more anodizing solutions having the formulas previously given. Thereafter, the aluminum metal is immersed in a finishing solution for a finishing anodizing and treatment. The current density to be applied may be higher than heretofore known, and the speed of the process is increased accordingly. The different anodizing baths provide different properties to the successive oxide coatings. These can be selected to provide the properties desired in accordance with those recited above.

The finishing bath may be followed, if desired, by any of the well-known baths employed for imparting color to the aluminum. If desired, furthermore, the pores of the oxide coating may be sealed by soaking the aluminum wire or strip in a sealing bath which can either follow the finishing bath or the coloring bath. One such sealing bath may be, for example, a solution of sodium sulphate or calcium sulphate, up to 40% by weight, nickel sulphate up to 10% by weight, the remainder of the solution being water. The solution is maintained at a temperature of 200 F. and the soaking period for maximum sealing effect to be obtained is for one minute. Another suitable sealing bath may be a solution of 2% by weight of polyethylene glycol and the remainder being water. At 200 F. the solution also provides a maximum sealing effect in one minute.

The following is provided, illustrating specific examples of the bath solutions and the electrical current employed.

First bath:

20% by weight of sulfuric acid, 66 Be. 1% wetting agent of quaternary ammonium compounds 1% by weight methyl cellulose 1% by weight oleic acid 3% by weight oxalic acid Second bath:

20% by weight sulfuric acid 2% by weight sodium chloride 5% by weight oxalic acid Third bath:

20% by weight sulfuric acid 2% by weight methyl cellulose 5% by weight disodium phosphate 1%by weight oleic acid The temperature was maintained at 70 F. Alternating cur-"rent was used in each bath, with a current density of either 160 amperes in 15 seconds per dm. or 40 amperes per drn. for one minute. 30 AWG gauge EC grade aluminum wire was treated. In he second example, all percentages are by weight:

Bath I Percent Sulfuric acid Oxalic acid 5 Wetting agent 1 Glycerine 2 .A.C. current employed.

Bath 11 Sulfuric acid 45 Glycerine M 5 Sodium chloride 1 DC. current was employed; tank was made positive.

Bath Ill Oxalic acid 5 Methyl cellulose 2 DE. current was employed; tank was made negative.

Bath IV Oxalic acid 5 Sodium phosphate 2 Methyl cellulose 2 AC. current was employed.

All the above in water. The temperature was maintained at 90 F. The current density was 300 amperes per square decimeter for 20 seconds. The third example employed 15% by weight of sulfuric acid, 5% by weight of oxalic acid, and 1% of a wetting agent for the composition for the first bath. The second, third, and fourth baths were the same as in the second example. Alternating current only was employed. A temperature of 50 F. was maintained. The current employed was 60 amperes per square decimeter for thirty seconds.

Typical of results obtained with the treatment indicated in the first example is shown 'by testing ten turns of twisted wire for voltage breakdown. It required 325 volts to cause breakdown at C., 360 volts at 300 C. When the product obtained in accordance with the first example was sealed in hot water, an even higher resistance was obtained. When the product obtained in accordance with the second example had a silicon insulation material impregnated into the pores, there was a still greater increase in the breakdown voltage required.

There has accordingly been described and shown herein a novel and useful method and means for anodizing aluminum which results not only in an improved oxide coating for the aluminum which is harder and more fiexible and which has a higher insulation resistance than those obtainable heretofore, but also is a much faster operation than heretofore possible. Furthermore, with the bath adjustments described, the ultimate properties de sired may be obtained by building up successive layers in every bath through which the aluminum is passed.

I claim:

1. A continuous method of providing an oxide coating on an elongated length of aluminum comprising immersing said aluminum successively and continuously first in a cleaning solution including a wetting agent, sulphuric acid, an oxalic acid, second in at least one anodizing solution and third in a finishing solution consisting of sulfuric acid, one of the phosphates selected from the group including disodium phosphate, and tetrasodium pyrophosphate, and methyl cellulose, causing an anodizing electrical current to flow through all said solutions with said aluminum eing the anode, and thereafter immersing said aluminum in a sealing solution.

2. A continuous method of providing an oxide coating as recited in claim 1 wherein said electrical current flow has an intensity at substantially 160 amperes per square decimeter for fifteen seconds at a temperature of F.

3. A continuous method of providing an oxide coating on an elongated length of aluminum as recited in claim 1 wherein the composition of the anodizing solution in which said elongated length of aluminum is immersed second comprises, by weight, 10 to 40% of sulfuric acid, 2 to 5% of oxalic acid, 1 to 10% of methyl cellulose, 0.5 to 5 of oleic acid, 2 to 20% of glycerine, and a wetting agent up to 1%.

4. A continuous method of providing an oxide coating on an elongated length of aluminum comprising immersing said aluminum successively to maintain conductive connection in a cleaning bath, then immersing said aluminum in an anodizing "bath including, by weight, 10 to 40% of sulfuric acid, 2 to 5% of oxalic acid, 2 to 10% of methyl cellulose, 0.5 to 5% of oleic acid, 2 to 20% of glyoerine, a wetting agent up to 1% by weight, and thereafter i-rmnersi-ng staid aluminum in a finishing solution including, by weight, 5 to 10% one of the phosphates selected from the group including disodium phosphate, and tetraso-dium pyrophosphate, 0.5 to 3% of methyl cellulose, and 10 to 40% of sulfuric acid, and applying electrical potential to cause a flow of electrical current in a circuit including said baths and said elongate-d length of aluminum with said aluminum always being the anode.

5. A continuous method of providing an oxide coating on an elongated length of film as. recited in claim 4 wherein the applied potential causes a current flow through said circuit having an intensity of substantially 300 amperes per decimeter square for twenty seconds at a temperature of substantially F.

6. A continuous method of providing an oxide coating on an elongated length of aluminum comprising immersing said aluminum successively and continuously first in a cleaning solution including a wetting agent, sulphuric acid and oxalic acid, second in an anodizing solution including 10 to 50 percent by weight of sulphuric acid, up to 2 percent by Weight of the chlorides chosen from magnesium chloride and sodium chloride, and up to 10 percent of glycerin, immersing said elongated length of aluminum in another anodizing solution including sulphuric acid from 10 to 50 percent by weight, up to 5 percent by weight of oxalic acid, immersing said elongated length of aluminum in still another anodizing solution including 10 to 50 percent by weight of sulphuric acid and 1 to 10 percent by weight of me-thylcellulose, finally immersing said elongated length of aluminum in a finishing solution including sulphuric acid, one of the phosphates selected from the group consisting of disodium phosphate and tetrasodium pyrophosphate, and methylcellulose, and causing an anodizing electrical current to flow through all of said solutions with said aluminum being the anode, and thereafter immersing said aluminum in a sealing solution.

References Citszi in the file of this patent UNITED STATES PATENTS 1,965,683 Work July 10, 1934 2,014,169 Edelman Sept. 10, 1935 2,122,392 Robinson et a1 June 28, 1938 8 Mason et a1 May 3, 1949 Simon et al Apr. 24, 1951 Sonnino Feb. 5, 1952 Mostovych et a1 Aug. 30, 1960 Ramirez Dec. 6, 1960 OTHER REFERENCES Ser. No. 369,618, Sassetti et a1. (A.P.C.), published May 18, 1943. 

1. A CONTINUOUS METHOD OF PROVIDING AN OXIDE COATING ON AN ELONGATED LENGTH OF ALUMINUM COMPRISING IMMERSING SAID ALUMINUM SUCCESSIVELY AND CONTINUOUSLY FIRST IN A CLEANING SOLUTION INCLUDING A WETTING AGENT, SULPHURIC ACID, AN OXALIC ACID, SECOND IN AT LEAST ONE ANODIZING SOLUTION AND THIRD IN A FINISHING SOLUTION CONSISTING OF SULFURIC ACID, ONE OF THE PHOSPHATES SELECTED FROM THE GROUP INCLUDING DISODIUM PHOSPHATE, AND TETRASODIUM PYROPHOSPHATE, AND METHYL CELLULOSE, CAUSING AN ANODIZING ELECTRICAL CURRENT TO FLOW THROUGH ALL SAID SOLUTIONS WITH SAID ALUMINUM BEING THE ANODE, AND THEREAFTER IMMERSING SAID ALUMINUM IN A SEALING SOLUTION. 